Thin film solar cells based on Cu(InGa)Se2 absorber layers have great promise for widespread application due to advantages including the highest efficiencies of any thin film solar cells, ability to be manufactured on low cost glass or flexible substrates, and demonstrated long-term stability. Research at the Institute of Energy Conversion and other laboratories worldwide has shown that polycrystalline thin film CuInSe2-based alloys are remarkable materials with wide compositional tolerance, inherently passive grain boundaries, and the ability to control the bandgap over the entire range of the solar spectrum by alloying with Ga or other elements.
At IEC, Cu(InGa)Se2 and other alloy films are deposited by thermal co-evaporation of the elements in either bell jar laboratory scale systems or in a pilot scale roll-to-roll system in which the film is continuously deposited on a moving flexible substrate. Alternatively, Cu(InGa)Se2 films are deposited using selenizaiton – the reaction of Cu-Ga-In metal precursor films in hydrogen serenade gas. A baseline process for complete cell fabrication is maintained and cells with efficiencies as high as 18% have been produced.
IEC has several current projects related to Cu(InGa)Se2 solar cells including:
Wide bandgap alloys
CuInSe2-based alloys with wider bandgap (Eg) provide potential advantages in the performance of cells and modules resulting from the tradeoff between higher voltage and lower current. Also, high efficiency wide bandgap devices are necessary for the development of tandem or multijunction devices. At IEC, we are investigating wide bandgap devices using Cu(InGa)Se2alloyed with high Ga content, Ag, and S.
Roll-to-roll deposition on flexible substrates
Scale-up of Cu(InGa)Se2 co-evaporation to manufacturing scale can be achieved with in-line deposition. At IEC, this is done in a roll-to-roll process on flexible (high temperature polyimide or metal foil) substrates. Research and development of in-line evaporation at IEC has focused on source design, process control and diagnostic tools, and material and process characterization.
Cu(InGa)(SeS)2 can be deposited by the reaction of metal precursors in a selenium continuing atmosphere. At IEC, Cu-Ga-In precursor films are deposited by sputtering and then reacted in hydride gases H2Se and H2S to form Cu(InGa)(SeS)2 films. Two-inch and six-inch diameter atmospheric pressure, tubular quartz reactors have been developed at IEC. Research efforts are focused on composition control, sodium addition, and use of different substrates in this process.
Recently IEC has added the capability to do laser and mechanical scribing on square foot size samples. This is being used to investigate monolithic integration for fabrication of Cu(InGa)Se2 modules on flexible substrates.
Alternative materials and device structures
Various alternative materials and device structures are under investigation at IEC. This includes sub-micron thick absorber layers, new flexible and rigid materials for substrates, encapsulation materials, and novel device structures including superstrate configurations. Many of these efforts are being done in collaboration with companies and other university research groups.
Selenization reactor for reaction of metal precursor films in H2Se or H2S for formation of Cu(InGa)(SeS)2.
IEC’s in-line co-evaporation system for roll-to-roll deposition of Cu(InGa)Se2 on a flexible substrate and a Cu(InGa)Se2 film deposited on a 100 ft long polyimide web.